In some circumstances it is desired to capture flow direction and velocity data in the hostile flow field produced from the dynamic pressure within the rapid expansion of gas products. Such an environment is commonly produced by explosive detonations. The environment experienced during a detonation inside of a multi-room structure comprises not only the shock and blast loading characterized by a very rapid release of energy, but also the fragmentation from the bomb casing material and the associated fragmentation of the internal structure and its associated contents. As the expanding gas flow moves from regions of confined high pressures to lower pressures, an energy wavefront from the detonation, as well as physical debris in the flow field, makes it difficult to use traditional “clean flow” instrumentation, such as hot-wire anemometry, pitot-tube or other conventional flow measurement methods. Thus, there has been a need to provide a sensor capable of operating in the hostile environment of detonation events.
The flow conditions inherent during an internal structure weapon detonation event are far from ideal for existing sensor technologies. The rapid expansion of gas products cause high velocity gas movement within the multi-room structure that is driven by the high-to-low pressure mechanism referred to as dynamic pressure. As the event unfolds, the detonation shock wavefront propagates through the structure in the form of a high temperature gas flow carrying the remnants of the explosive case, debris within the structure, and test specific artifacts (desks, chairs, containers, shelves, etc.) at high speeds. Testing has demonstrated that this flow field is far from clean and contains massive amounts of high temperature gas/particulates and debris. Further, the hostile environment destroys sensors and produces zero visibility. It is in this extreme condition that existing traditional sensors experience shortcomings.
Dynamic pressure sensors currently in use at DTRA test facilities typically use Wheatstone bridge type circuitry to measure pressure conditions using pitot-static probes such as the DTRA XCW-8WN-200 probe. This type of sensor measures along a streamline and produces unidirectional dynamic pressure for subsonic single-phase flow conditions. The nature of the sensing ports makes it susceptible to contamination from particulate matter and physical damage from debris impact. To provide any level of directionality to this type of measurement system requires many such probes to be oriented in the flow with a substantial investment in calibration of the probes. Multi-hole pressure probes can provide up to a 70° receptivity flow angle, but require 5-7 pressure transducers to achieve that cone of flow directionality. They too suffer from the contamination and debris issues mentioned for the current pitot-static tube systems, and have the requirement for dry and non-reactive gases.
Another suite of existing sensor technologies that initially appears attractive but suffer from the hostile environment and the zero visibility are those that use laser array attenuation, particle image velocimetry or other visible tracking techniques. In addition to the technical difficulties in deploying systems of this type, cost considerations necessarily preclude these solutions from being cost-effective approaches.
Measuring wind speeds through the use of piezoelectric load sensors has been accomplished in the past. For example, U.S. Pat. No. 4,615,214, issued Oct. 7, 1986, entitled, Piezoelectric Wind Sensor, provides an array of piezoelectric sensors mounted around the circumference of a vertical shaft, as disclosed in FIGS. 1 and 2 of the patent. The shaft is forced against the sensors by the wind, and by observing the direction of greatest force magnitude, the direction of the wind can be determined. The speed of the wind is also determined through the force readings by inserting the largest force reading into a look up table that is calibrated to read velocity.
However, many piezoelectric crystals are needed to realize a velocity vector in the '214 design, and the piezoelectric components are exposed to the elements, which make its use in detonation environments problematic.
In U.S. Pat. No. 4,366,718, issued Jan. 4, 1983, entitled “Bi Directional Flow Transducer”, fluid flows through a restriction core mounted in the center of cylindrical packaging. Flow impacts the restriction core and causes it to slide along the centerline of the outer casing in the direction of fluid flow. The flow restricting core deflects relative to the spring constant of the two movement restricting springs mounted on either side of it. The restriction core is attached to a probe that deflects axially along with the core. A linear differential voltage transducer (LVDT) senses the probe's deflection and produces an electrical profiling of the spring's contraction. However, LVDTs are too large to use in sensors used to measure 2D flow fields, as desired herein. In fact, two units would be needed, and the size of the casing would be directly proportional to how accurate the LVDTs were.
In U.S. Pat. No. 4,332,157, issued Jun. 1, 1982, entitled “Pyroelectric Anemometer Concept”, two pyroelectric sensors sandwich a heating element. In no wind conditions, the heating element affects both sensors the same, and each are a fluctuating median temperature. In windy conditions, the upstream sensor is cooled, while downstream sensor is heated due to the wind forcing more convective heat transfer from the heating element to the downstream sensor than in the no wind condition. This type of design is not applicable in sensing detonation events, as heating is not an optimal means by which to gather data on explosive wind events due to their quick duration and the necessity of equilibrium. Differential temperatures between the sensors would still exist no matter how hot the explosive event, but with extremely high wind temperatures, the ability to measure differential temperatures diminishes and would be costly. Further, this type of sensor has no ability to resolve direction.
U.S. Pat. No. 4,905,513, issued Mar. 6, 1990, entitled “Wind Speed Measuring Device”, the temperature difference between the heated coil and the casing of the sensor is measured with the difference being the change in temperature due to the wind. This differential temperature is processed to compute wind speed, while wind direction is realized by processing the signals of each wire around the periphery of the cylinder and computing the direction of the largest gradient. With such a design, extreme wind conditions could have adverse effect on small wires, and certain processing must be dedicated to accounting for changes in ambient temperature. Accordingly, fatigue and embrittlement may cause inaccurate readings over time.
In U.S. Pat. No. 3,408,855, issued on Nov. 5, 1968, entitled “Apparatus for determining detonation velocity of explosives”, a sensor is provided wherein the pressure of a detonation event collapses the conductive outer shell over a length of resistive coiled wires. The change in resistance of the wires is used to mathematically derive the velocity of air hitting the sensor. This sensor, however, is solely based on pressure, and the effect of temperature on the resistivity of wire is not accounted for. Further, the outer casing permanently collapses upon each detonation, and is therefore not reusable.
A current commercial one dimensional air blast sensor being used to measure detonations is the LC33 Canadian piezoelectric instrument (DTIC ADA302543), which is a pencil model which has a sensitive element consisting of a short cylinder of lead zirconate titanate with a sensitivity of 120 pc/psi. Testing of this sensor has shown it to be problematic, possibly due to stressing in sensitive elements. It displayed unsatisfactory performance in detonation tests conducted under the Monograph Air Blast Instrumentation (MABS) project.
The MQ10 British piezoelectric instrument, illustrated in FIG. 1, is another commercially available sensor. It is comprised of a quartz crystal with a hatched-shaped streamline baffle and sensitivity of 100 pc/psi from 1 to 70 psi. The MQ10 gage is ranged from 1 to 300 psi. It usually mounted in a concrete block, flush with ground surface.
Although this gage gave the nearest approximation to true pressure-time variations in blast wave of all gage types deployed in the Monograph Air Blast Instrumentation (MABS) project, this device was not designed for multiaxial applications, and post processing is thus necessary to derive wind velocity.
The commercially available KKQ American piezoelectric instrument, illustrated in FIG. 2, uses piezo-electric elements to measure dynamic pressures by observing the difference in stagnation and side on pressures. This gage showed promising results in the Monograph Air Blast Instrumentation (MABS) tests, and was the only gage presented that had the ability to directly determine wind speed. However, it does not resolve direction, and is in effect a piezo-electric, pitot-static probe.
Other such devices have been patented with similar characteristics as those above. In the case of sensing wind speed from a detonation event, it would be impractical to use small wires for durability reasons unless properly shielded. The extremely brief test durations would limit the possibility of thermodynamic equilibrium occurring between resistive wires and the gas flow, therefore thermodynamic metal expansion is inapplicable.
A probe based sensor system that has the fidelity to measure gas flow velocity and direction yet survive the hostile environment of a detonation event would be desirable.